Gfats as modifiers of the p53 pathway and methods of use

ABSTRACT

Human GFAT genes are identified as modulators of the p53 pathway, and thus are therapeutic targets for disorders associated with defective p53 function. Methods for identifying modulators of p53, comprising screening for agents that modulate the activity of GFAT are provided.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applications60/296,076 filed Jun. 5, 2001, 60/328,605 filed Oct. 10, 2001, and60/357,253 filed Feb. 15, 2002. The contents of the prior applicationsare hereby incorporated in their entirety.

BACKGROUND OF THE INVENTION

The p53 gene is mutated in over 50 different types of human cancers,including familial and spontaneous cancers, and is believed to be themost commonly mutated gene in human cancer (Zambetti and Levine, FASEB(1993) 7:855-865; Hollstein, et al., Nucleic Acids Res. (1994)22:3551-3555). Greater than 90% of mutations in the p53 gene aremissense mutations that alter a single amino acid that inactivates p53function. Aberrant forms of human p53 are associated with poorprognosis, more aggressive tumors, metastasis, and short survival rates(Mitsudomi et al., Clin Cancer Res October 2000; 6(10):4055-63;Koshland, Science (1993) 262:1953).

The human p53 protein normally functions as a central integrator ofsignals including DNA damage, hypoxia, nucleotide deprivation, andoncogene activation (Prives, Cell (1998) 95:5-8). In response to thesesignals, p53 protein levels are greatly increased with the result thatthe accumulated p53 activates cell cycle arrest or apoptosis dependingon the nature and strength of these signals. Indeed, multiple lines ofexperimental evidence have pointed to a key role for p53 as a tumorsuppressor (Levine, Cell (1997) 88:323-331). For example, homozygous p53“knockout” mice are developmentally normal but exhibit nearly 100%incidence of neoplasia in the first year of life (Donehower et al.,Nature (1992) 356:215-221).

The biochemical mechanisms and pathways through which p53 functions innormal and cancerous cells are not fully understood, but one clearlyimportant aspect of p53 function is its activity as a gene-specifictranscriptional activator. Among the genes with known p53-responseelements are several with well-characterized roles in either regulationof the cell cycle or apoptosis, including GADD45, p21/Waf1/Cip1, cyclinG, Bax, IGF-BP3, and MDM2 (Levine, Cell (1997) 88:323-331).

Glutamine:fructose-6-phosphate amidotransferases (GFATs) are involved inthe hexosamine bisynthesis pathway. GFAT1 intiates the formation ofglucosamine 6-phosphate, the first step as well as the rate-limitingenzyme of the hexosamine biosyiithetic pathway (Traxinger, R., andMarshall, S. (1991) J. Biol. Chem. 266: 10148-10154). GFAT1 controls theflux of glucose into the hexosamine pathway, and therefore controls theformation of hexosamine products. GFAT1 is likely to be involved inregulating the availability of precursors for N— and O-linkedglycosylation of proteins (Robinson, A. et al. (1993) Diabetes. 42:1333-1346). It is an insulin-regulated enzyme, plays an important rolein the induction of insulin resistance in cultured cells, and isinvolved in the upregulation in kidney associated with diabeticnephropathy (Daniels, M. et al. (1996) J Clin Invest; 97(5): 1235-41).In MDA468 human breast cells, EGF stimulates the accumulation of GFATmessenger RNA (MRNA) to a level 4-fold higher than that in unstimulatedcells (Paterson A J, and Kudlow J E. (1995) Endocrinology136:2809-2816).

Glutamine-fructose-6-phosphate transaminase 2 (GFAT2 or GFPT2) formsglucosamine 6-phosphate by transferring the amide group from L-glutamineto fructose 6-phosphate in the synthesis of hexosamines.

The ability to manipulate the genomes of model organisms such asDrosophila provides a powerful means to analyze biochemical processesthat, due to significant evolutionary conservation, have directrelevance to more complex vertebrate organisms. Due to a high level ofgene and pathway conservation, the strong similarity of cellularprocesses, and the functional conservation of genes between these modelorganisms and mammals, identification of the involvement of novel genesin particular pathways and their functions in such model organisms candirectly contribute to the understanding of the correlative pathways andmethods of modulating them in mammals (see, for example, Mechler B M etal., 1985 EMBO J 4:1551-1557; Gateff E. 1982 Adv. Cancer Res. 37: 33-74;Watson K L., et al., 1994 J Cell Sci. 18: 19-33; Miklos G L, and Rubin GM. 1996 Cell 86:521-529; Wassarman D A, et al., 1995 Curr Opin Gen Dev5: 44-50; and Booth D R. 1999 Cancer Metastasis Rev. 18: 261-284). Forexample, a genetic screen can be carried out in an invertebrate modelorganism having underexpression (e.g. knockout) or overexpression of agene (referred to as a “genetic entry point”) that yields a visiblephenotype. Additional genes are mutated in a random or targeted manner.When a gene mutation changes the original phenotype caused by themutation in the genetic entry point, the gene is identified as a“modifier” involved in the same or overlapping pathway as the geneticentry point. When the genetic entry point is an ortholog of a human geneimplicated in a disease pathway, such as p53, modifier genes can beidentified that may be attractive candidate targets for noveltherapeutics.

All references cited herein, including sequence information inreferenced Genbank identifier numbers and website references, areincorporated herein in their entireties.

SUMMARY OF THE INVENTION

We have discovered genes that modify the p53 pathway in Drosophila, andidentified their human orthologs, hereinafter referred to as GFAT. Theinvention provides methods for utilizing these p53 modifier genes andpolypeptides to identify candidate therapeutic agents that can be usedin the treatment of disorders associated with defective p53 function.Preferred GFAT-modulating agents specifically bind to GFAT polypeptidesand restore p53 function. Other preferred GFAT-modulating agents arenucleic acid modulators such as antisense oligomers and RNAi thatrepress GFAT gene expression or product activity by, for example,binding to and inhibiting the respective nucleic acid (i.e. DNA orMRNA).

GFAT-specific modulating agents may be evaluated by any convenient invitro or in vivo assay for molecular interaction with a GFAT polypeptideor nucleic acid. In one embodiment, candidate p53 modulating agents aretested with an assay system comprising a GFAT polypeptide or nucleicacid. Candidate agents that produce a change in the activity of theassay system relative to controls are identified as candidate p53modulating agents. The assay system may be cell-based or cell-free.GFAT-modulating agents include GFAT related proteins (e.g. dominantnegative mutants, and biotherapeutics); GFAT-specific antibodies;GFAT-specific antisense oligomers and other nucleic acid modulators; andchemical agents that specifically bind GFAT or compete with GFAT bindingtarget. In one specific embodiment, a small molecule modulator isidentified using a transferase assay. In specific embodiments, thescreening assay system is selected from a binding assay, an apoptosisassay, a cell proliferation assay, an angiogenesis assay, and a hypoxicinduction assay.

In another embodiment, candidate p53 pathway modulating agents arefurther tested using a second assay system that detects changes in thep53 pathway, such as angiogenic, apoptotic, or cell proliferationchanges produced by the originally identified candidate agent or anagent derived from the original agent. The second assay system may usecultured cells or non-human animals. In specific embodiments, thesecondary assay system uses non-human animals, including animalspredetermined to have a disease or disorder implicating the p53 pathway,such as an angiogenic, apoptotic, or cell proliferation disorder (e.g.cancer).

The invention further provides methods for modulating the p53 pathway ina mammalian cell by contacting the mammalian cell with an agent thatspecifically binds a GFAT polypeptide or nucleic acid. The agent may bea small molecule modulator, a nucleic acid modulator, or an antibody andmay be administered to a mammalian animal predetermined to have apathology associated the p53 pathway.

DETAILED DESCRIPTION OF THE INVENTION

Genetic screens were designed to identify modifiers of the p53 pathwayin Drosophila in which p53 was overexpressed in the wing (Ollmann M, etal., Cell 2000 101: 91-101). The CG1345 gene was identified as amodifier of the p53 pathway. Accordingly, vertebrate orthologs of thesemodifiers, and preferably the human orthologs, glutamine fructose 6phosphate transaminase (GFAT) genes (i.e., nucleic acids andpolypeptides) are attractive drug targets for the treatment ofpathologies associated with a defective p53 signaling pathway, such ascancer.

In vitro and in vivo methods of assessing GFAT function are providedherein. Modulation of the GFAT or their respective binding partners isuseful for understanding the association of the p53 pathway and itsmembers in normal and disease conditions and for developing diagnosticsand therapeutic modalities for p53 related pathologies. GFAT-modulatingagents that act by inhibiting or enhancing GFAT expression, directly orindirectly, for example, by affecting a GFAT function such as enzymatic(e.g., catalytic) or binding activity, can be identified using methodsprovided herein. GFAT modulating agents are useful in diagnosis, therapyand pharmaceutical development.

Nucleic Acids and Polypeptides of the Invention

Sequences related to GFAT nucleic acids and polypeptides that can beused in the invention are disclosed in Genbank (referenced by Genbankidentifier (GI) number) as GI#s 183081 (SEQ ID NO:1), 4503980 (SEQ IDNO:2), 4826741 (SEQ ID NO:3), 10433934 (SEQ ID NO:4), and 12652544 (SEQID NO:5) for nucleic acid, and GI#s 544382 (SEQ ID NO:6), 4503981 (SEQID NO:7), and 4826742 (SEQ ID NO:8) for polypeptides.

GFATs are transferase proteins with amidotransferase and sugar isomerase(SIS) domains. The term “GFAT polypeptide” refers to a full-length GFATprotein or a functionally active fragment or derivative thereof. A“functionally active” GFAT fragment or derivative exhibits one or morefunctional activities associated with a full-length, wild-type GFATprotein, such as antigenic or immunogenic activity, enzymatic activity,ability to bind natural cellular substrates, etc. The functionalactivity of GFAT proteins, derivatives and fragments can be assayed byvarious methods known to one skilled in the art (Current Protocols inProtein Science (1998) Coligan et al., eds., John Wiley & Sons, Inc.,Somerset, N.J.) and as further discussed below. For purposes herein,functionally active fragments also include those fragments that compriseone or more structural domains of a GFAT, such as a transferase domainor a binding domain. Protein domains can be identified using the PFAMprogram (Bateman A., et al., Nucleic Acids Res, 1999, 27:260-2;http://pfam.wustl.edu). For example, the Glutamine amidotransferasesclass-II domains (PFAM 00310) of GFATs from GI#s 544382 (SEQ ID NO:6)and 4826742 (SEQ ID NO:8) are located at approximately amino acidresidues 2-210 and 2-207, respectively. Methods for obtaining GFATpolypeptides are also further described below. In some embodiments,preferred fragments are functionally active, domain-containing fragmentscomprising at least 25 contiguous amino acids, preferably at least 50,more preferably 75, and most preferably at least 100 contiguous aminoacids of any one of SEQ ID NOs:6, 7, or 8 (a GFAT). In further preferredembodiments, the fragment comprises the entire transferase or SIS(functionally active) domain.

The term “GFAT nucleic acid” refers to a DNA or RNA molecule thatencodes a GFAT polypeptide. Preferably, the GFAT polypeptide or nucleicacid or fragment thereof is from a human, but can also be an ortholog,or derivative thereof with at least 70% sequence identity, preferably atleast 80%, more preferably 85%, still more preferably 90%, and mostpreferably at least 95% sequence identity with GFAT. Normally, orthologsin different species retain the same function, due to presence of one ormore protein motifs and/or 3-dimensional structures. Orthologs aregenerally identified by sequence homology analysis, such as BLASTanalysis, usually using protein bait sequences. Sequences are assignedas a potential ortholog if the best hit sequence from the forward BLASTresult retrieves the original query sequence in the reverse BLAST(Huynen M A and Bork P, Proc Natl Acad Sci (1998) 95:5849-5856; Huynen MA et al., Genome Research (2000) 10:1204-1210). Programs for multiplesequence alignment, such as CLUSTAL (Thompson J D et al, 1994, NucleicAcids Res 22:4673-4680) may be used to highlight conserved regionsand/or residues of orthologous proteins and to generate phylogenetictrees. In a phylogenetic tree representing multiple homologous sequencesfrom diverse species (e.g., retrieved through BLAST analysis),orthologous sequences from two species generally appear closest on thetree with respect to all other sequences from these two species.Structural threading or other analysis of protein folding (e.g., usingsoftware by ProCeryon, Biosciences, Salzburg, Austria) may also identifypotential orthologs. In evolution, when a gene duplication event followsspeciation, a single gene in one species, such as Drosophila, maycorrespond to multiple genes (paralogs) in another, such as human. Asused herein, the term “orthologs” encompasses paralogs. As used herein,“percent (%) sequence identity” with respect to a subject sequence, or aspecified portion of a subject sequence, is defined as the percentage ofnucleotides or amino acids in the candidate derivative sequenceidentical with the nucleotides or amino acids in the subject sequence(or specified portion thereof), after aligning the sequences andintroducing gaps, if necessary to achieve the maximum percent sequenceidentity, as generated by the program WU-BLAST-2.0a19 (Altschul et al.,J. Mol. Biol. (1997) 215:403-410;http://blast.wustl.edu/blast/README.html) with all the search parametersset to default values. The HSP S and HSP S2 parameters are dynamicvalues and are established by the program itself depending upon thecomposition of the particular sequence and composition of the particulardatabase against which the sequence of interest is being searched. A %identity value is determined by the number of matching identicalnucleotides or amino acids divided by the sequence length for which thepercent identity is being reported. “Percent (%) amino acid sequencesimilarity” is determined by doing the same calculation as fordetermining % amino acid sequence identity, but including conservativeamino acid substitutions in addition to identical amino acids in thecomputation.

A conservative amino acid substitution is one in which an amino acid issubstituted for another amino acid having similar properties such thatthe folding or activity of the protein is not significantly affected.Aromatic amino acids that can be substituted for each other arephenylalanine, tryptophan, and tyrosine; interchangeable hydrophobicamino acids are leucine, isoleucine, methionine, and valine;interchangeable polar amino acids are glutamine and asparagine;interchangeable basic amino acids are arginine, lysine and histidine;interchangeable acidic amino acids are aspartic acid and glutamic acid;and interchangeable small amino acids are alanine, serine, threonine,cysteine and glycine.

Alternatively, an alignment for nucleic acid sequences is provided bythe local homology algorithm of Smith and Waterman (Smith and Waterman,1981, Advances in Applied Mathematics 2:482-489; database: EuropeanBioinformatics Institute http://www.ebi.ac.uk/MPsrch/; Smith andWaterman, 1981, J. of Molec. Biol., 147:195-197; Nicholas et al., 1998,“A Tutorial on Searching Sequence Databases and Sequence ScoringMethods” (www.psc.edu) and references cited therein.; W. R. Pearson,1991, Genomics 11:635-650). This algorithm can be applied to amino acidsequences by using the scoring matrix developed by Dayhoff (Dayhoff:Atlas of Protein Sequences and Structure, M. O. Dayhoff ed., 5 suppl.3:353-358, National Biomedical Research Foundation, Washington, D.C.,USA), and normalized by Gribskov (Gribskov 1986 Nucl. Acids Res.14(6):6745-6763). The Smith-Waterman algorithm may be employed wheredefault parameters are used for scoring (for example, gap open penaltyof 12, gap extension penalty of two). From the data generated, the“Match” value reflects “sequence identity.”

Derivative nucleic acid molecules of the subject nucleic acid moleculesinclude sequences that hybridize to the nucleic acid sequence of any ofSEQ ID NOs:1, 2, 3, 4, or 5. The stringency of hybridization can becontrolled by temperature, ionic strength, pH, and the presence ofdenaturing agents such as formamide during hybridization and washing.Conditions routinely used are set out in readily available proceduretexts (e.g., Current Protocol in Molecular Biology, Vol. 1, Chap. 2.10,John Wiley & Sons, Publishers (1994); Sambrook et al., MolecularCloning, Cold Spring Harbor (1989)). In some embodiments, a nucleic acidmolecule of the invention is capable of hybridizing to a nucleic acidmolecule containing the nucleotide sequence of any one of SEQ ID NOs:1,2, 3, 4, or 5 under stringent hybridization conditions that comprise:prehybridization of filters containing nucleic acid for 8 hours toovernight at 65° C. in a solution comprising 6×single strength citrate(SSC) (1×SSC is 0.15 M NaCl, 0.015 M Na citrate; pH 7.0), 5× Denhardt'ssolution, 0.05% sodium pyrophosphate and 100 μg/ml herring sperm DNA;hybridization for 18-20 hours at 65° C. in a solution containing 6×SSC,1× Denhardt's solution, 100 μg/ml yeast tRNA and 0.05% sodiumpyrophosphate; and washing of filters at 65° C. for 1 h in a solutioncontaining 0.2×SSC and 0.1% SDS (sodium dodecyl sulfate).

In other embodiments, moderately stringent hybridization conditions areused that comprise: pretreatment of filters containing nucleic acid for6 h at 40° C. in a solution containing 35% formamide, 5×SSC, 50 mMTris-HCl (pH 7.5), 5 mM EDTA, 0.1% PVP, 0.1% Ficoll, 1% BSA, and 500μg/ml denatured salmon sperm DNA; hybridization for 18-20 h at 40° C. ina solution containing 35% formamide, 5×SSC, 50 mM Tris-HCl (pH 7.5), 5mM EDTA, 0.02% PVP, 0.02% Ficoll, 0.2% BSA, 100 μg/ml salmon sperm DNA,and 10% (wt/vol) dextran sulfate; followed by washing twice for 1 hourat 55°0 C. in a solution containing 2×SSC and 0.1% SDS.

Alternatively, low stringency conditions can be used that comprise:incubation for 8 hours to overnight at 37° C. in a solution comprising20% formamide, 5×SSC, 50 nM sodium phosphate (pH 7.6), 5× Denhardt'ssolution, 10% dextran sulfate, and 20 μg/ml denatured sheared salmonsperm DNA; hybridization in the same buffer for 18 to 20 hours; andwashing of filters in 1×SSC at about 37° C. for 1 hour.

Isolation, Production, Expression, and Mis-expression of GFAT NucleicAcids and Polypeptides

GFAT nucleic acids and polypeptides, useful for identifying and testingagents that modulate GFAT function and for other applications related tothe involvement of GFAT in the p53 pathway. GFAT nucleic acids andderivatives and orthologs thereof may be obtained using any availablemethod. For instance, techniques for isolating cDNA or genomic DNAsequences of interest by screening DNA libraries or by using polymerasechain reaction (PCR) are well known in the art. In general, theparticular use for the protein will dictate the particulars ofexpression, production, and purification methods. For instance,production of proteins for use in screening for modulating agents mayrequire methods that preserve specific biological activities of theseproteins, whereas production of proteins for antibody generation mayrequire structural integrity of particular epitopes. Expression ofproteins to be purified for screening or antibody production may requirethe addition of specific tags (e.g., generation of fusion proteins).Overexpression of a GFAT protein for assays used to assess GFATfunction, such as involvement in cell cycle regulation or hypoxicresponse, may require expression in eukaryotic cell lines capable ofthese cellular activities. Techniques for the expression, production,and purification of proteins are well known in the art; any suitablemeans therefore may be used (e.g., Higgins S J and Hames B D (eds.)Protein Expression: A Practical Approach, Oxford University Press Inc.,New York 1999; Stanbury PF et al., Principles of FermentationTechnology, 2^(nd) edition, Elsevier Science, New York, 1995; Doonan S(ed.) Protein Purification Protocols, Humana Press, New Jersey, 1996;Coligan J E et al, Current Protocols in Protein Science (eds.), 1999,John Wiley & Sons, New York). In particular embodiments, recombinantGFAT is expressed in a cell line known to have defective p53 function(e.g. SAOS-2 osteoblasts, H1299 lung cancer cells, C33A and HT3 cervicalcancer cells, HT-29 and DLD-1 colon cancer cells, among others,available from American Type Culture Collection (ATCC), Manassas, Va.).The recombinant cells are used in cell-based screening assay systems ofthe invention, as described further below.

The nucleotide sequence encoding a GFAT polypeptide can be inserted intoany appropriate expression vector. The necessary transcriptional andtranslational signals, including promoter/enhancer element, can derivefrom the native GFAT gene and/or its flanking regions or can beheterologous. A variety of host-vector expression systems may beutilized, such as mammalian cell systems infected with virus (e.g.vaccinia virus, adenovirus, etc.); insect cell systems infected withvirus (e.g. baculovirus); microorganisms such as yeast containing yeastvectors, or bacteria transformed with bacteriophage, plasmid, or cosmidDNA. A host cell strain that modulates the expression of, modifies,and/or specifically processes the gene product may be used.

To detect expression of the GFAT gene product, the expression vector cancomprise a promoter operably linked to a GFAT gene nucleic acid, one ormore origins of replication, and, one or more selectable markers (e.g.thymidine kinase activity, resistance to antibiotics, etc.).Alternatively, recombinant expression vectors can be identified byassaying for the expression of the GFAT gene product based on thephysical or functional properties of the GFAT protein in in vitro assaysystems (e.g. immunoassays).

The GFAT protein, fragment, or derivative may be optionally expressed asa fusion, or chimeric protein product (i.e. it is joined via a peptidebond to a heterologous protein sequence of a different protein), forexample to facilitate purification or detection. A chimeric product canbe made by ligating the appropriate nucleic acid sequences encoding thedesired amino acid sequences to each other using standard methods andexpressing the chimeric product. A chimeric product may also be made byprotein synthetic techniques, e.g. by use of a peptide synthesizer(Hunkapiller et al., Nature (1984) 310:105-111).

Once a recombinant cell that expresses the GFAT gene sequence isidentified, the gene product can be isolated and purified using standardmethods (e.g. ion exchange, affinity, and gel exclusion chromatography;centrifugation; differential solubility; electrophoresis, citepurification reference). Alternatively, native GFAT proteins can bepurified from natural sources, by standard methods (e.g. immunoaffinitypurification). Once a protein is obtained, it may be quantified and itsactivity measured by appropriate methods, such as immunoassay, bioassay,or other measurements of physical properties, such as crystallography.

The methods of this invention may also use cells that have beenengineered for altered expression (mis-expression) of GFAT or othergenes associated with the p53 pathway. As used herein, mis-expressionencompasses ectopic expression, over-expression, under-expression, andnon-expression (e.g. by gene knock-out or blocking expression that wouldotherwise normally occur).

Genetically Modified Animals

Animal models that have been genetically modified to alter GFATexpression may be used in in vivo assays to test for activity of acandidate p53 modulating agent, or to further assess the role of GFAT ina p53 pathway process such as apoptosis or cell proliferation.Preferably, the altered GFAT expression results in a detectablephenotype, such as decreased or increased levels of cell proliferation,angiogenesis, or apoptosis compared to control animals having normalGFAT expression. The genetically modified animal may additionally havealtered p53 expression (e.g. p53 knockout). Preferred geneticallymodified animals are mammals such as primates, rodents (preferablymice), cows, horses, goats, sheep, pigs, dogs and cats. Preferrednon-mammalian species include zebrafish, C. elegans, and Drosophila.Preferred genetically modified animals are transgenic animals having aheterologous nucleic acid sequence present as an extrachromosomalelement in a portion of its cells, i.e. mosaic animals (see, forexample, techniques described by Jakobovits, 1994, Curr. Biol.4:761-763.) or stably integrated into its germ line DNA (i.e., in thegenomic sequence of most or all of its cells): Heterologous nucleic acidis introduced into the germ line of such transgenic animals by geneticmanipulation of, for example, embryos or embryonic stem cells of thehost animal.

Methods of making transgenic animals are well-known in the art (fortransgenic mice see Brinster et al., Proc. Nat. Acad. Sci. USA 82:4438-4442 (1985), U.S. Pat. Nos. 4,736,866 and 4,870,009, both by Lederet al., U.S. Pat. No. 4,873,191 by Wagner et al., and Hogan, B.,Manipulating the Mouse Embryo, Cold Spring Harbor Laboratory Press, ColdSpring Harbor, N.Y., (1986); for particle bombardment see U.S. Pat. No.,4,945,050, by Sandford et al.; for transgenic Drosophila see Rubin andSpradling, Science (1982) 218:348-53 and U.S. Pat. No. 4,670,388; fortransgenic insects see Berghammer A. J. et al., A Universal Marker forTransgenic Insects (1999) Nature 402:370-371; for transgenic Zebrafishsee Lin S., Transgenic Zebrafish, Methods Mol Biol. (2000);136:375-3830); for microinjection procedures for fish, amphibian eggsand birds see Houdebine and Chourrout, Experientia (1991) 47:897-905;for transgenic rats see Hammer et al., Cell (1990) 63:1099-1112; and forculturing of embryonic stem (ES) cells and the subsequent production oftransgenic animals by the introduction of DNA into ES cells usingmethods such as electroporation, calcium phosphate/DNA precipitation anddirect injection see, e.g., Teratocarcinomas and Embryonic Stem Cells, APractical Approach, E. J. Robertson, ed., IRL Press (1987)). Clones ofthe nonhuman transgenic animals can be produced according to availablemethods (see Wilmut, I. et al. (1997) Nature 385:810-813; and PCTInternational Publication Nos. WO 97/07668 and WO 97/07669).

In one embodiment, the transgenic animal is a “knock-out” animal havinga heterozygous or homozygous alteration in the sequence of an endogenousGFAT gene that results in a decrease of GFAT function, preferably suchthat GFAT expression is undetectable or insignificant. Knock-out animalsare typically generated by homologous recombination with a vectorcomprising a transgene having at least a portion of the gene to beknocked out. Typically a deletion, addition or substitution has beenintroduced into the transgene to functionally disrupt it. The transgenecan be a human gene (e.g., from a human genomic clone) but morepreferably is an ortholog of the human gene derived from the transgenichost species. For example, a mouse GFAT gene is used to construct ahomologous recombination vector suitable for altering an endogenous GFATgene in the mouse genome. Detailed methodologies for homologousrecombination in mice are available (see Capecchi, Science (1989)244:1288-1292; Joyner et al., Nature (1989) 338:153-156). Procedures forthe production of non-rodent transgenic mammals and other animals arealso available (Houdebine and Chourrout, supra; Pursel et al., Science(1989) 244:1281-1288; Simms et al., Bio/Technology (1988) 6:179-183). Ina preferred embodiment, knock-out animals, such as mice harboring aknockout of a specific gene, may be used to produce antibodies againstthe human counterpart of the gene that has been knocked out (Claesson MH et al., (1994) Scan J Immunol 40:257-264; Declerck P J et al., (1995)J Biol Chem. 270:8397-400).

In another embodiment, the transgenic animal is a “knock-in” animalhaving an alteration in its genome that results in altered expression(e.g., increased (including ectopic) or decreased expression) of theGFAT gene, e.g., by introduction of additional copies of GFAT, or byoperatively inserting a regulatory sequence that provides for alteredexpression of an endogenous copy of the GFAT gene. Such regulatorysequences include inducible, tissue-specific, and constitutive promotersand enhancer elements. The knock-in can be homozygous or heterozygous.

Transgenic nonhuman animals can also be produced that contain selectedsystems allowing for regulated expression of the transgene. One exampleof such a system that may be produced is the cre/loxP recombinase systemof bacteriophage P1 (Lakso et al., PNAS (1992) 89:6232-6236; U.S. Pat.No. 4,959,317). If a cre/loxP recombinase system is used to regulateexpression of the transgene, animals containing transgenes encoding boththe Cre recombinase and a selected protein are required. Such animalscan be provided through the construction of “double” transgenic animals,e.g., by mating two transgenic animals, one containing a transgeneencoding a selected protein and the other containing a transgeneencoding a recombinase. Another example of a recombinase system is theFLP recombinase system of Saccharomyces cerevisiae (O'Gorman et al.(1991) Science 251:1351-1355; U.S. Pat. No. 5,654,182). In a preferredembodiment, both Cre-LoxP and Flp-Frt are used in the same system toregulate expression of the transgene, and for sequential deletion ofvector sequences in the same cell (Sun X et al (2000) Nat Genet25:83-6).

The genetically modified animals can be used in genetic studies tofurther elucidate the p53 pathway, as animal models of disease anddisorders implicating defective p53 function, and for in vivo testing ofcandidate therapeutic agents, such as those identified in screensdescribed below. The candidate therapeutic agents are administered to agenetically modified animal having altered GFAT function and phenotypicchanges are compared with appropriate control animals such asgenetically modified animals that receive placebo treatment, and/oranimals with unaltered GFAT expression that receive candidatetherapeutic agent.

In addition to the above-described genetically modified animals havingaltered GFAT function, animal models having defective p53 function (andotherwise normal GFAT function), can be used in the methods of thepresent invention. For example, a p53 knockout mouse can be used toassess, in vivo, the activity of a candidate p53 modulating agentidentified in one of the in vitro assays described below. p53 knockoutmice are described in the literature (Jacks et al., Nature2001;410:1111-1116, 1043-1044; Donehower et al., supra). Preferably, thecandidate p53 modulating agent when administered to a model system withcells defective in p53 function, produces a detectable phenotypic changein the model system indicating that the p53 function is restored, i.e.,the cells exhibit normal cell cycle progression.

Modulating Agents

The invention provides methods to identify agents that interact withand/or modulate the function of GFAT and/or the p53 pathway. Such agentsare useful in a variety of diagnostic and therapeutic applicationsassociated with the p53 pathway, as well as in further analysis of theGFAT protein and its contribution to the p53 pathway. Accordingly, theinvention also provides methods for modulating the p53 pathwaycomprising the step of specifically modulating GFAT activity byadministering a GFAT-interacting or -modulating agent.

In a preferred embodiment, GFAT-modulating agents inhibit or enhanceGFAT activity or otherwise affect normal GFAT function, includingtranscription, protein expression, protein localization, and cellular orextra-cellular activity. In a further preferred embodiment, thecandidate p53 pathway-modulating agent specifically modulates thefunction of the GFAT. The phrases “specific modulating agent”,“specifically modulates”, etc., are used herein to refer to modulatingagents that directly bind to the GFAT polypeptide or nucleic acid, andpreferably inhibit, enhance, or otherwise alter, the function of theGFAT. The term also encompasses modulating agents that alter theinteraction of the GFAT with a binding partner or substrate (e.g. bybinding to a binding partner of a GFAT, or to a protein/binding partnercomplex, and inhibiting function).

Preferred GFAT-modulating agents include small molecule compounds;GFAT-interacting proteins, including antibodies and otherbiotherapeutics; and nucleic acid modulators such as antisense and RNAinhibitors. The modulating agents may be formulated in pharmaceuticalcompositions, for example, as compositions that may comprise otheractive ingredients, as in combination therapy, and/or suitable carriersor excipients. Techniques for formulation and administration of thecompounds may be found in “Remington's Pharmaceutical Sciences” MackPublishing Co., Easton, Pa., 19^(th) edition.

Small Molecule Modulators

Small molecules, are often preferred to modulate function of proteinswith enzymatic function, and/or containing protein interaction domains.Chemical agents, referred to in the art as “small molecule” compoundsare typically organic, non-peptide molecules, having a molecular weightless than 10,000, preferably less than 5,000, more preferably less than1,000, and most preferably less than 500. This class of modulatorsincludes chemically synthesized molecules, for instance, compounds fromcombinatorial chemical libraries. Synthetic compounds may be rationallydesigned or identified based on known or inferred properties of the GFATprotein or may be identified by screening compound libraries.Alternative appropriate modulators of this class are natural products,particularly secondary metabolites from organisms such as plants orfungi, which can also be identified by screening compound libraries forGFAT-modulating activity. Methods for generating and obtaining compoundsare well known in the art (Schreiber S L, Science (2000) 151: 1964-1969;Radmann J and Gunther J, Science (2000) 151:1947-1948).

Small molecule modulators identified from screening assays, as describedbelow, can be used as lead compounds from which candidate clinicalcompounds may be designed, optimized, and synthesized. Such clinicalcompounds may have utility in treating pathologies associated with thep53 pathway. The activity of candidate small molecule modulating agentsmay be improved several-fold through iterative secondary functionalvalidation, as further described below, structure determination, andcandidate modulator modification and testing. Additionally, candidateclinical compounds are generated with specific regard to clinical andpharmacological properties. For example, the reagents may be derivatizedand re-screened using in vitro and in vivo assays to optimize activityand minimize toxicity for pharmaceutical development.

Protein Modulators

Specific GFAT-interacting proteins are useful in a variety of diagnosticand therapeutic applications related to the p53 pathway and relateddisorders, as well as in validation assays for other GFAT-modulatingagents. In a preferred embodiment, GFAT-interacting proteins affectnormal GFAT function, including transcription, protein expression,protein localization, and cellular or extra-cellular activity. Inanother embodiment, GFAT-interacting proteins are useful in detectingand providing information about the function of GFAT proteins, as isrelevant to p53 related disorders, such as cancer (e.g., for diagnosticmeans).

A GFAT-interacting protein may be endogenous, i.e. one that naturallyinteracts genetically or biochemically with a GFAT, such as a member ofthe GFAT pathway that modulates GFAT expression, localization, and/oractivity. GFAT-modulators include dominant negative forms ofGFAT-interacting proteins and of GFAT proteins themselves. Yeasttwo-hybrid and variant screens offer preferred methods for identifyingendogenous GFAT-interacting proteins (Finley, R. L. et al. (1996) in DNACloning-Expression Systems: A Practical Approach, eds. Glover D. & HamesB. D (Oxford University Press, Oxford, England), pp. 169-203; Fashema SF et al., Gene (2000) 250:1-14; Drees B L Curr Opin Chem Biol (1999)3:64-70; Vidal M and Legrain P Nucleic Acids Res (1999) 27:919-29; andU.S. Pat. No. 5,928,868). Mass spectrometry is an alternative preferredmethod for the elucidation of protein complexes (reviewed in, e.g.,Pandley A and Mann M, Nature (2000) 405:837-846; Yates J R 3^(rd),Trends Genet (2000) 16:5-8).

A GFAT-interacting protein may be an exogenous protein, such as aGFAT-specific antibody or a T-cell antigen receptor (see, e.g., Harlowand Lane (1988) Antibodies, A Laboratory Manual, Cold Spring HarborLaboratory; Harlow and Lane (1999) Using antibodies: a laboratorymanual. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press).GFAT antibodies are further discussed below.

In preferred embodiments, a GFAT-interacting protein specifically bindsa GFAT protein. In alternative preferred embodiments, a GFAT-modulatingagent binds a GFAT substrate, binding partner, or cofactor.

Antibodies

In another embodiment, the protein modulator is a GFAT specific antibodyagonist or antagonist. The antibodies have therapeutic and diagnosticutilities, and can be used in screening assays to identify GFATmodulators. The antibodies can also be used in dissecting the portionsof the GFAT pathway responsible for various cellular responses and inthe general processing and maturation of the GFAT.

Antibodies that specifically bind GFAT polypeptides can be generatedusing known methods. Preferably the antibody is specific to a mammalianortholog of GFAT polypeptide, and more preferably, to human GFAT.Antibodies may be polyclonal, monoclonal (mAbs), humanized or chimericantibodies, single chain antibodies, Fab fragments, F(ab′).sub.2fragments, fragments produced by a FAb expression library,anti-idiotypic (anti-Id) antibodies, and epitope-binding fragments ofany of the above. Epitopes of GFAT which are particularly antigenic canbe selected, for example, by routine screening of GFAT polypeptides forantigenicity or by applying a theoretical method for selecting antigenicregions of a protein (Hopp and Wood (1981), Proc. Nati. Acad. Sci.U.S.A. 78:3824-28; Hopp and Wood, (1983) Mol. Immunol. 20:483-89;Sutcliffe et al., (1983) Science 219:660-66) to the amino acid sequenceshown in any of SEQ ID NOs:6, 7, or 8. Monoclonal antibodies withaffinities of 10⁸ M⁻¹ preferably 10⁹ M⁻¹ to 10¹⁰ M⁻¹, or stronger can bemade by standard procedures as described (Harlow and Lane, supra; Goding(1986) Monoclonal Antibodies: Principles and Practice (2d ed) AcademicPress, New York; and U.S. Pat. Nos. 4,381,292; 4,451,570; and4,618,577). Antibodies may be generated against crude cell extracts ofGFAT or substantially purified fragments thereof. If GFAT fragments areused, they preferably comprise at least 10, and more preferably, atleast 20 contiguous amino acids of a GFAT protein. In a particularembodiment, GFAT-specific antigens and/or immunogens are coupled tocarrier proteins that stimulate the immune response. For example, thesubject polypeptides are covalently coupled to the keyhole limpethemocyanin (KLH) carrier, and the conjugate is emulsified in Freund'scomplete adjuvant, which enhances the immune response. An appropriateimmune system such as a laboratory rabbit or mouse is immunizedaccording to conventional protocols.

The presence of GFAT-specific antibodies is assayed by an appropriateassay such as a solid phase enzyme-linked immunosorbant assay (ELISA)using immobilized corresponding GFAT polypeptides. Other assays, such asradioimmunoassays or fluorescent assays might also be used.

Chimeric antibodies specific to GFAT polypeptides can be made thatcontain different portions from different animal species. For instance,a human immunoglobulin constant region may be linked to a variableregion of a murine mAb, such that the antibody derives its biologicalactivity from the human antibody, and its binding specificity from themurine fragment. Chimeric antibodies are produced by splicing togethergenes that encode the appropriate regions from each species (Morrison etal., Proc. Natl. Acad. Sci. (1984) 81:6851-6855; Neuberger et al.,Nature (1984) 312:604-608; Takeda et al., Nature (1985) 31:452-454).Humanized antibodies, which are a form of chimeric antibodies, can begenerated by grafting complementary-determining regions (CDRs) (Carlos,T. M., J. M. Harlan. 1994. Blood 84:2068-2101) of mouse antibodies intoa background of human framework regions and constant regions byrecombinant DNA technology (Riechmann L M, et al., 1988 Nature 323:323-327). Humanized antibodies contain ˜10% murine sequences and ˜90%human sequences, and thus further reduce or eliminate immunogenicity,while retaining the antibody specificities (Co M S, and Queen C. 1991Nature 351: 501-501; Morrison S L. 1992 Ann. Rev. Immun. 10:239-265).Humanized antibodies and methods of their production are well-known inthe art (U.S. Pat. Nos. 5,530,101, 5,585,089, 5,693,762, and 6,180,370).

GFAT-specific single chain antibodies which are recombinant, singlechain polypeptides formed by linking the heavy and light chain fragmentsof the Fv regions via an amino acid bridge, can be produced by methodsknown in the art (U.S. Pat. No. 4,946,778; Bird, Science (1988)242:423-426; Huston et al., Proc. Natl. Acad. Sci. USA (1988)85:5879-5883; and Ward et al., Nature (1989) 334:544-546).

Other suitable techniques for antibody production involve in vitroexposure of lymphocytes to the antigenic polypeptides or alternativelyto selection of libraries of antibodies in phage or similar vectors(Huse et al., Science (1989) 246:1275-1281). As used herein, T-cellantigen receptors are included within the scope of antibody modulators(Harlow and Lane, 1988, supra).

The polypeptides and antibodies of the present invention may be usedwith or without modification. Frequently, antibodies will be labeled byjoining, either covalently or non-covalently, a substance that providesfor a detectable signal, or that is toxic to cells that express thetargeted protein (Menard S, et al., Int J. Biol Markers (1989)4:131-134). A wide variety of labels and conjugation techniques areknown and are reported extensively in both the scientific and patentliterature. Suitable labels include radionuclides, enzymes, substrates,cofactors, inhibitors, fluorescent moieties, fluorescent emittinglanthanide metals, chemiluminescent moieties, bioluminescent moieties,magnetic particles, and the like (U.S. Pat. Nos. 3,817,837; 3,850,752;3,939,350; 3,996,345; 4,277,437; 4,275,149; and 4,366,241). Also,recombinant immunoglobulins may be produced (U.S. Pat. No. 4,816,567).Antibodies to cytoplasmic polypeptides may be delivered and reach theirtargets by conjugation with membrane-penetrating toxin proteins (U.S.Pat. No. 6,086,900).

When used therapeutically in a patient, the antibodies of the subjectinvention are typically administered parenterally, when possible at thetarget site, or intravenously. The therapeutically effective dose anddosage regimen is determined by clinical studies. Typically, the amountof antibody administered is in the range of about 0.1 mg/kg-to about 10mg/kg of patient weight. For parenteral administration, the antibodiesare formulated in a unit dosage injectable form (e.g., solution,suspension, emulsion) in association with a pharmaceutically acceptablevehicle. Such vehicles are inherently nontoxic and non-therapeutic.Examples are water, saline, Ringer's solution, dextrose solution, and 5%human serum albumin. Nonaqueous vehicles such as fixed oils, ethyloleate, or liposome carriers may also be used. The vehicle may containminor amounts of additives, such as buffers and preservatives, whichenhance isotonicity and chemical stability or otherwise enhancetherapeutic potential. The antibodies' concentrations in such vehiclesare typically in the range of about 1 mg/ml to about 10 mg/ml.Immunotherapeutic methods are further described in the literature (USPat. No. 5,859,206; WO0073469).

Nucleic Acid Modulators

Other preferred GFAT-modulating agents comprise nucleic acid molecules,such as antisense oligomers or double stranded RNA (dsRNA), whichgenerally inhibit GFAT activity. Preferred nucleic acid modulatorsinterfere with the function of the GFAT nucleic acid such as DNAreplication, transcription, translocation of the GFAT RNA to the site ofprotein translation, translation of protein from the GFAT RNA, splicingof the GFAT RNA to yield one or more mRNA species, or catalytic activitywhich may be engaged in or facilitated by the GFAT RNA.

In one embodiment, the antisense oligomer is an oligonucleotide that issufficiently complementary to a GFAT mRNA to bind to and preventtranslation, preferably by binding to the 5′ untranslated region.GFAT-specific antisense oligonucleotides, preferably range from at least6 to about 200 nucleotides. In some embodiments the oligonucleotide ispreferably at least 10, 15, or 20 nucleotides in length. In otherembodiments, the oligonucleotide is preferably less than 50, 40, or 30nucleotides in length. The oligonucleotide can be DNA or RNA or achimeric mixture or derivatives or modified versions thereof,single-stranded or double-stranded. The oligonucleotide can be modifiedat the base moiety, sugar moiety, or phosphate backbone. Theoligonucleotide may include other appending groups such as peptides,agents that facilitate transport across the cell membrane,hybridization-triggered cleavage agents, and intercalating agents.

In another embodiment, the antisense oligomer is a phosphothioatemorpholino oligomer (PMO). PMOs are assembled from four differentmorpholino subunits, each of which contain one of four genetic bases (A,C, G, or T) linked to a six-membered morpholine ring. Polymers of thesesubunits are joined by non-ionic phosphodiamidate intersubunit linkages.Details of how to make and use PMOs and other antisense oligomers arewell known in the art (e.g. see WO99/18193; Probst J C, AntisenseOligodeoxynucleotide and Ribozyme Design, Methods. (2000) 22(3):271-281;Summerton J, and Weller D. 1997 Antisense Nucleic Acid DrugDev.:7:187-95; U.S. Pat. No. 5,235,033; and U.S. Pat No. 5,378,841).

Alternative preferred GFAT nucleic acid modulators are double-strandedRNA species mediating RNA interference (RNAi). RNAi is the process ofsequence-specific, post-transcriptional gene silencing in animals andplants, initiated by double-stranded RNA (dsRNA) that is homologous insequence to the silenced gene. Methods relating to the use of RNAi tosilence genes in C. elegans, Drosophila, plants, and humans are known inthe art (Fire A, et al., 1998 Nature 391:806-811; Fire, A. Trends Genet.15, 358-363 (1999); Sharp, P. A. RNA interference 2001. Genes Dev. 15,485-490 (2001); Hammond, S. M., et al., Nature Rev. Genet. 2, 110-1119(2001); Tuschl, T. Chem. Biochem. 2, 239-245 (2001); Hamilton, A. etal., Science 286, 950-952 (1999); Hammond, S. M., et al., Nature 404,293-296 (2000); Zamore, P. D., et al., Cell 101, 25-33 (2000);Bernstein, E., et al., Nature 409, 363-366 (2001); Elbashir, S. M., etal., Genes Dev. 15, 188-200 (2001); WO0129058; WO9932619; Elbashir S M,et al., 2001 Nature 411:494-498).

Nucleic acid modulators are commonly used as research reagents,diagnostics, and therapeutics. For example, antisense oligonucleotides,which are able to inhibit gene expression with exquisite specificity,are often used to elucidate the function of particular genes (see, forexample, U.S. Pat. No. 6,165,790). Nucleic acid modulators are alsoused, for example, to distinguish between functions of various membersof a biological pathway. For example, antisense oligomers have beenemployed as therapeutic moieties in the treatment of disease states inanimals and man and have been demonstrated in numerous clinical trialsto be safe and effective (Milligan J F, et al, Current Concepts inAntisense Drug Design, J Med Chem. (1993) 36:1923-1937; Tonkinson J L etal., Antisense Oligodeoxynucleotides as Clinical Therapeutic Agents,Cancer Invest. (1996) 14:54-65). Accordingly, in one aspect of theinvention, a GFAT-specific nucleic acid modulator is used in an assay tofurther elucidate the role of the GFAT in the p53 pathway, and/or itsrelationship to other members of the pathway. In another aspect of theinvention, a GFAT-specific antisense oligomer is used as a therapeuticagent for treatment of p53-related disease states.

Assay Systems

The invention provides assay systems and screening methods foridentifying specific modulators of GFAT activity. As used herein, an“assay system” encompasses all the components required for performingand analyzing results of an assay that detects and/or measures aparticular event. In general, primary assays are used to identify orconfirm a modulator's specific biochemical or molecular effect withrespect to the GFAT nucleic acid or protein. In general, secondaryassays further assess the activity of a GFAT modulating agent identifiedby a primary assay and may confirm that the modulating agent affectsGFAT in a manner relevant to the p53 pathway. In some cases, GFATmodulators will be directly tested in a secondary assay.

In a preferred embodiment, the screening method comprises contacting asuitable assay system comprising a GFAT polypeptide with a candidateagent under conditions whereby, but for the presence of the agent, thesystem provides a reference activity (e.g. transferase activity), whichis based on the particular molecular event the screening method detects.A statistically significant difference between the agent-biased activityand the reference activity indicates that the candidate agent modulatesGFAT activity, and hence the p53 pathway.

Primary Assays

The type of modulator tested generally determines the type of primaryassay.

Primary Assays for Small Molecule Modulators

For small molecule modulators, screening assays are used to identifycandidate modulators. Screening assays may be cell-based or may use acell-free system that recreates or retains the relevant biochemicalreaction of the target protein (reviewed in Sittampalam GS et aL, CurrOpin Chem Biol (1997) 1:384-91 and accompanying references). As usedherein the term “cell-based” refers to assays using live cells, deadcells, or a particular cellular fraction, such as a membrane,endoplasmic reticulum, or mitochondrial fraction. The term “cell free”encompasses assays using substantially purified protein (eitherendogenous or recombinantly produced), partially purified or crudecellular extracts. Screening assays may detect a variety of molecularevents, including protein-DNA interactions, protein-protein interactions(e.g., receptor-ligand binding), transcriptional activity (e.g., using areporter gene), enzymatic activity (e.g., via a property of thesubstrate), activity of second messengers, immunogenicty and changes incellular morphology or other cellular characteristics. Appropriatescreening assays may use a wide range of detection methods includingfluorescent, radioactive, colorimetric, spectrophotometric, andamperometric methods, to provide a read-out for the particular molecularevent detected.

Cell-based screening assays usually require systems for recombinantexpression of GFAT and any auxiliary proteins demanded by the particularassay. Appropriate methods for generating recombinant proteins producesufficient quantities of proteins that retain their relevant biologicalactivities and are of sufficient purity to optimize activity and assureassay reproducibility. Yeast two-hybrid and variant screens, and massspectrometry provide preferred methods for determining protein-proteininteractions and elucidation of protein complexes. In certainapplications, when GFAT-interacting proteins are used in screens toidentify small molecule modulators, the binding specificity of theinteracting protein to the GFAT protein may be assayed by various knownmethods such as substrate processing (e.g. ability of the candidateGFAT-specific binding agents to function as negative effectors inGFAT-expressing cells), binding equilibrium constants (usually at leastabout 10⁷ M⁻¹, preferably at least about 10⁸ M⁻¹, more preferably atleast about 10⁹ M⁻¹), and immunogenicity (e.g. ability to elicit GFATspecific antibody in a heterologous host such as a mouse, rat, goat orrabbit). For enzymes and receptors, binding may be assayed by,respectively, substrate and ligand processing.

The screening assay may measure a candidate agent's ability tospecifically bind to or modulate activity of a GFAT polypeptide, afusion protein thereof, or to cells or membranes bearing the polypeptideor fusion protein. The GFAT polypeptide can be full length or a fragmentthereof that retains functional GFAT activity. The GFAT polypeptide maybe fused to another polypeptide, such as a peptide tag for detection oranchoring, or to another tag. The GFAT polypeptide is preferably humanGFAT, or is an ortholog or derivative thereof as described above. In apreferred embodiment, the screening assay detects candidate agent-basedmodulation of GFAT interaction with a binding target, such as anendogenous or exogenous protein or other substrate that hasGFAT-specific binding activity, and can be used to assess normal GFATgene function.

Suitable assay formats that may be adapted to screen for GFAT modulatorsare known in the art. Preferred screening assays are high throughput orultra high throughput and thus provide automated, cost-effective meansof screening compound libraries for lead compounds (Fernandes PB, CurrOpin Chem Biol (1998) 2:597-603; Sundberg S A, Curr Opin Biotechnol2000, 11:47-53). In one preferred embodiment, screening assays usesfluorescence technologies, including fluorescence polarization,time-resolved fluorescence, and fluorescence resonance energy transfer.These systems offer means to monitor protein-protein or DNA-proteininteractions in which the intensity of the signal emitted fromdye-labeled molecules depends upon their interactions with partnermolecules (e.g., Selvin P R, Nat Struct Biol (2000) 7:730-4; Fernandes PB, supra; Hertzberg R P and Pope A J, Curr Opin Chem Biol (2000)4:445-451).

A variety of suitable assay systems may be used to identify candidateGFAT and p53 pathway modulators (e.g. U.S. Pat. Nos. 5,550,019 and6,133,437 (apoptosis assays); U.S. Pat. No. 6,020,135 (p53 modulation),among others). GFAT activity may be measured spectrophotometrically,using fructose-6-phosphate, glutamine, and 3-acetylpyridine adeninedinucleotide as substrates. Here, the change of absorbance resultingfrom reduction of 3-acetylpyridine adenine dinucleotide is monitoredspectrophotometrically (McKnight, G. L et al (1992) J. Biol. Chem. 267:25208-25212; Traxinger, R. R., and Marshall, S. (1991) J. Biol. Chem.266:10148-10154).

Apoptosis assays. Assays for apoptosis may be performed by terminaldeoxynucleotidyl transferase-mediated digoxigenin-11-dUTP nick endlabeling (TUNEL) assay. The TUNEL assay is used to measure nuclear DNAfragmentation characteristic of apoptosis ( Lazebnik et al., 1994,Nature 371, 346), by following the incorporation of fluorescein-dUTP(Yonehara et al., 1989, J. Exp. Med. 169, 1747). Apoptosis may furtherbe assayed by acridine orange staining of tissue culture cells (Lucas,R., et al., 1998, Blood 15:4730-41). An apoptosis assay system maycomprise a cell that expresses a GFAT, and that optionally has defectivep53 function (e.g. p53 is over-expressed or under-expressed relative towild-type cells). A test agent can be added to the apoptosis assaysystem and changes in induction of apoptosis relative to controls whereno test agent is added, identify candidate p53 modulating agents. Insome embodiments of the invention, an apoptosis assay may be used as asecondary assay to test a candidate p53 modulating agents that isinitially identified using a cell-free assay system. An apoptosis assaymay also be used to test whether GFAT function plays a direct role inapoptosis. For example, an apoptosis assay may be performed on cellsthat over- or under-express GFAT relative to wild type cells.Differences in apoptotic response compared to wild type cells suggeststhat the GFAT plays a direct role in the apoptotic response. Apoptosisassays are described further in U.S. Pat. No. 6,133,437.

Cell proliferation and cell cycle assays. Cell proliferation may beassayed via bromodeoxyuridine (BRDU) incorporation. This assayidentifies a cell population undergoing DNA synthesis by incorporationof BRDU into newly-synthesized DNA. Newly-synthesized DNA may then bedetected using an anti-BRDU antibody (Hoshino et al., 1986, Int. J.Cancer 38, 369; Campana et al., 1988, J. Immunol. Meth. 107, 79), or byother means.

Cell Proliferation may also be examined using [³H]-thymidineincorporation (Chen, J., 1996, Oncogene 13:1395-403; Jeoung, J., 1995,J. Biol. Chem. 270:18367-73). This assay allows for quantitativecharacterization of S-phase DNA syntheses. In this assay, cellssynthesizing DNA will incorporate [³H]-thymidine into newly synthesizedDNA. Incorporation can then be measured by standard techniques such asby counting of radioisotope in a scintillation counter (e.g., Beckman LS 3800 Liquid Scintillation Counter).

Cell proliferation may also be assayed by colony formation in soft agar(Sambrook et al., Molecular Cloning, Cold Spring Harbor (1989)). Forexample, cells transformed with GFAT are seeded in soft agar plates, andcolonies are measured and counted after two weeks incubation.

Involvement of a gene in the cell cycle may be assayed by flow cytometry(Gray J W et al. (1986) Int J Radiat Biol Relat Stud Phys Chem Med49:237-55). Cells transfected with a GFAT may be stained with propidiumiodide and evaluated in a flow cytometer (available from BectonDickinson).

Accordingly, a cell proliferation or cell cycle assay system maycomprise a cell that expresses a GFAT, and that optionally has defectivep53 function (e.g. p53 is over-rxpressed or under-expressed relative towild-type cells). A test agent can be added to the assay system andchanges in cell proliferation or cell cycle relative to controls whereno test agent is added, identify candidate p53 modulating agents. Insome embodiments of the invention, the cell proliferation or cell cycleassay may be used as a secondary assay to test a candidate p53modulating agents that is initially identified using another assaysystem such as a cell-free assay system. A cell proliferation assay mayalso be used to test whether GFAT function plays a direct role in cellproliferation or cell cycle. For example, a cell proliferation or cellcycle assay may be performed on cells that over- or under-express GFATrelative to wild type cells. Differences in proliferation or cell cyclecompared to wild type cells suggests that the GFAT plays a direct rolein cell proliferation or cell cycle.

Angiogenesis. Angiogenesis may be assayed using various humanendothelial cell systems, such as umbilical vein, coronary artery, ordermal cells. Suitable assays include Alamar Blue based assays(available from Biosource International) to measure proliferation;migration assays using fluorescent molecules, such as the use of BectonDickinson Falcon HTS FluoroBlock cell culture inserts to measuremigration of cells through membranes in presence or absence ofangiogenesis enhancer or suppressors; and tubule formation assays basedon the formation of tubular structures by endothelial cells on Matrigel®(Becton Dickinson). Accordingly, an angiogenesis assay system maycomprise a cell that expresses a GFAT, and that optionally has defectivep53 function (e.g. p53 is over-expressed or under-expressed relative towild-type cells). A test agent can be added to the angiogenesis assaysystem and changes in angiogenesis relative to controls where no testagent is added, identify candidate p53 modulating agents. In someembodiments of the invention, the angiogenesis assay may be used as asecondary assay to test a candidate p53 modulating agents that isinitially identified using another assay system. An angiogenesis assaymay also be used to test whether GFAT function plays a direct role incell proliferation. For example, an angiogenesis assay may be performedon cells that over- or under-express GFAT relative to wild type cells.Differences in angiogenesis compared to wild type cells suggests thatthe GFAT plays a direct role in angiogenesis.

Hypoxic induction. The alpha subunit of the transcription factor,hypoxia inducible factor-1 (HIF-1), is upregulated in tumor cellsfollowing exposure to hypoxia in vitro. Under hypoxic conditions, HIF-1stimulates the expression of genes known to be important in tumour cellsurvival, such as those encoding glyolytic enzymes and VEGF. Inductionof such genes by hypoxic conditions may be assayed by growing cellstransfected with GFAT in hypoxic conditions (such as with 0.1% O2, 5%CO2, and balance N2, generated in a Napco 7001 incubator (PrecisionScientific)) and normoxic conditions, followed by assessment of geneactivity or expression by Taqman®. For example, a hypoxic inductionassay system may comprise a cell that expresses a GFAT, and thatoptionally has a mutated p53 (e.g. p53 is over-expressed orunder-expressed relative to wild-type cells). A test agent can be addedto the hypoxic induction assay system and changes in hypoxic responserelative to controls where no test agent is added, identify candidatep53 modulating agents. In some embodiments of the invention, the hypoxicinduction assay may be used as a secondary assay to test a candidate p53modulating agents that is initially identified using another assaysystem. A hypoxic induction assay may also be used to test whether GFATfunction plays a direct role in the hypoxic response. For example, ahypoxic induction assay may be performed on cells that over- orunder-express GFAT relative to wild type cells. Differences in hypoxicresponse compared to wild type cells suggests that the GFAT plays adirect role in hypoxic induction.

Cell adhesion. Cell adhesion assays measure adhesion of cells topurified adhesion proteins, or adhesion of cells to each other, inpresence or absence of candidate modulating agents. Cell-proteinadhesion assays measure the ability of agents to modulate the adhesionof cells to purified proteins. For example, recombinant proteins areproduced, diluted to 2.5 g/mL in PBS, and used to coat the wells of amicrotiter plate. The wells used for negative control are not coated.Coated wells are then washed, blocked with 1% BSA, and washed again.Compounds are diluted to 2× final test concentration and added to theblocked, coated wells. Cells are then added to the wells, and theunbound cells are washed off. Retained cells are labeled directly on theplate by adding a membrane-permeable fluorescent dye, such ascalcein-AM, and the signal is quantified in a fluorescent microplatereader.

Cell-cell adhesion assays measure the ability of agents to modulatebinding of cell adhesion proteins with their native ligands. Theseassays use cells that naturally or recombinantly express the adhesionprotein of choice. In an exemplary assay, cells expressing the celladhesion protein are plated in wells of a multiwell plate. Cellsexpressing the ligand are labeled with a membrane-permeable fluorescentdye, such as BCECF, and allowed to adhere to the monolayers in thepresence of candidate agents. Unbound cells are washed off, and boundcells are detected using a fluorescence plate reader.

High-throughput cell adhesion assays have also been described. In onesuch assay, small molecule ligands and peptides are bound to the surfaceof microscope slides using a microarray spotter, intact cells are thencontacted with the slides, and unbound cells are washed off. In thisassay, not only the binding specificity of the peptides and modulatorsagainst cell lines are determined, but also the functional cellsignaling of attached cells using immunofluorescence techniques in situon the microchip is measured (Falsey J R et al., Bioconjug Chem.May-June 2001;12(3):346-53).

Primary Assays for Antibody Modulators

For antibody modulators, appropriate primary assays test is a bindingassay that tests the antibody's affinity to and specificity for the GFATprotein. Methods for testing antibody affinity and specificity are wellknown in the art (Harlow and Lane, 1988, 1999, supra). The enzyme-linkedimmunosorbant assay (ELISA) is a preferred method for detectingGFAT-specific antibodies; others include FACS assays, radioimmunoassays,and fluorescent assays.

Primary Assays for Nucleic Acid Modulators

For nucleic acid modulators, primary assays may test the ability of thenucleic acid modulator to inhibit or enhance GFAT gene expression,preferably MRNA expression. In general, expression analysis comprisescomparing GFAT expression in like populations of cells (e.g., two poolsof cells that endogenously or recombinantly express GFAT) in thepresence and absence of the nucleic acid modulator. Methods foranalyzing mRNA and protein expression are well known in the art. Forinstance, Northern blotting, slot blotting, ribonuclease protection,quantitative RT-PCR (e.g., using the TaqMan®, PE Applied Biosystems), ormicroarray analysis may be used to confirm that GFAT mRNA expression isreduced in cells treated with the nucleic acid modulator (e.g., CurrentProtocols in Molecular Biology (1994) Ausubel F M et al., eds., JohnWiley & Sons, Inc., chapter 4; Freeman W M et al., Biotechniques (1999)26:112-125; Kallioniemi O P, Ann Med 2001, 33:142-147; Blohm D H andGuiseppi-Elie, A Curr Opin Biotechnol 2001, 12:41-47). Proteinexpression may also be monitored. Proteins are most commonly detectedwith specific antibodies or antisera directed against either the GFATprotein or specific peptides. A variety of means including Westernblotting, ELISA, or in situ detection, are available (Harlow E and LaneD, 1988 and 1999, supra).

Secondary Assays

Secondary assays may be used to further assess the activity ofGFAT-modulating agent identified by any of the above methods to confirmthat the modulating agent affects GFAT in a manner relevant to the p53pathway. As used herein, GFAT-modulating agents encompass candidateclinical compounds or other agents derived from previously identifiedmodulating agent. Secondary assays can also be used to test the activityof a modulating agent on a particular genetic or biochemical pathway orto test the specificity of the modulating agent's interaction with GFAT.

Secondary assays generally compare like populations of cells or animals(e.g., two pools of cells or animals that endogenously or recombinantlyexpress GFAT) in the presence and absence of the candidate modulator. Ingeneral, such assays test whether treatment of cells or animals with acandidate GFAT-modulating agent results in changes in the p53 pathway incomparison to untreated (or mock- or placebo-treated) cells or animals.Certain assays use “sensitized genetic backgrounds”, which, as usedherein, describe cells or animals engineered for altered expression ofgenes in the p53 or interacting pathways.

Cell-Based Assays

Cell based assays may use a variety of mammalian cell lines known tohave defective p53 function (e.g. SAOS-2 osteoblasts, H1299 lung cancercells, C33A and HT3 cervical cancer cells, HT-29 and DLD-1 colon cancercells, among others, available from American Type Culture Collection(ATCC), Manassas, Va.). Cell based assays may detect endogenous p53pathway activity or may rely on recombinant expression of p53 pathwaycomponents. Any of the aforementioned assays may be used in thiscell-based format. Candidate modulators are typically added to the cellmedia but may also be injected into cells or delivered by any otherefficacious means.

Animal Assays

A variety of non-human animal models of normal or defective p53 pathwaymay be used to test candidate GFAT modulators. Models for defective p53pathway typically use genetically modified animals that have beenengineered to mis-express (e.g., over-express or lack expression in)genes involved in the p53 pathway. Assays generally require systemicdelivery of the candidate modulators, such as by oral administration,injection, etc.

In a preferred embodiment, p53 pathway activity is assessed bymonitoring neovascularization and angiogenesis. Animal models withdefective and normal p53 are used to test the candidate modulator'saffect on GFAT in Matrigel® assays. Matrigel® is an extract of basementmembrane proteins, and is composed primarily of laminin, collagen IV,and heparin sulfate proteoglycan. It is provided as a sterile liquid at4°0 C., but rapidly forms a solid gel at 37° C. Liquid Matrigel® ismixed with various angiogenic agents, such as bFGF and VEGF, or withhuman tumor cells which over-express the GFAT. The mixture is theninjected subcutaneously(SC) into female athymic nude mice (Taconic,Germantown, N.Y.) to support an intense vascular response. Mice withMatrigel® pellets may be dosed via oral (PO), intraperitoneal (IP), orintravenous (IV) routes with the candidate modulator. Mice areeuthanized 5-12 days post-injection, and the Matrigel® pellet isharvested for hemoglobin analysis (Sigma plasma hemoglobin kit).Hemoglobin content of the gel is found to correlate the degree ofneovascularization in the gel.

In another preferred embodiment, the effect of the candidate modulatoron GFAT is assessed via tumorigenicity assays. In one example, xenografthuman tumors are implanted SC into female athymic mice, 6-7 week old, assingle cell suspensions either from a pre-existing tumor or from invitro culture. The tumors which express the GFAT endogenously areinjected in the flank, 1×10⁵ to 1×10⁷ cells per mouse in a volume of 100μL using a 27 gauge needle. Mice are then ear tagged and tumors aremeasured twice weekly. Candidate modulator treatment is initiated on theday the mean tumor weight reaches 100 mg. Candidate modulator isdelivered IV, SC, IP, or PO by bolus administration. Depending upon thepharmacokinetics of each unique candidate modulator, dosing can beperformed multiple times per day. The tumor weight is assessed bymeasuring perpendicular diameters with a caliper and calculated bymultiplying the measurements of diameters in two dimensions. At the endof the experiment, the excised tumors maybe utilized for biomarkeridentification or further analyses. For immunohistochemistry staining,xenograft tumors are fixed in 4% paraformaldehyde, 0.1M phosphate, pH7.2, for 6 hours at 4° C., immersed in 30% sucrose in PBS, and rapidlyfrozen in isopentane cooled with liquid nitrogen.

Diagnostic and Therapeutic Uses

Specific GFAT-modulating agents are useful in a variety of diagnosticand therapeutic applications where disease or disease prognosis isrelated to defects in the p53 pathway, such as angiogenic, apoptotic, orcell proliferation disorders. Accordingly, the invention also providesmethods for modulating the p53 pathway in a cell, preferably a cellpre-determined to have defective p53 function, comprising the step ofadministering an agent to the cell that specifically modulates GFATactivity. Preferably, the modulating agent produces a detectablephenotypic change in the cell indicating that the p53 function isrestored, i.e., for example, the cell undergoes normal proliferation orprogression through the cell cycle.

The discovery that GFAT is implicated in p53 pathway provides for avariety of methods that can be employed for the diagnostic andprognostic evaluation of diseases and disorders involving defects in thep53 pathway and for the identification of subjects having apredisposition to such diseases and disorders.

Various expression analysis methods can be used to diagnose whether GFATexpression occurs in a particular sample, including Northern blotting,slot blotting, ribonuclease protection, quantitative RT-PCR, andmicroarray analysis. (e.g., Current Protocols in Molecular Biology(1994) Ausubel F M et al., eds., John Wiley & Sons, Inc., chapter 4;Freeman W M et al., Biotechniques (1999) 26:112-125; Kallioniemi O P,Ann Med 2001, 33:142-147; Blohm and Guiseppi-Elie, Curr Opin Biotechnol2001, 12:41-47). Tissues having a disease or disorder implicatingdefective p53 signaling that express a GFAT, are identified as amenableto treatment with a GFAT modulating agent. In a preferred application,the p53 defective tissue overexpresses a GFAT relative to normal tissue.For example, a Northern blot analysis of mRNA from tumor and normal celllines, or from tumor and matching normal tissue samples from the samepatient, using full or partial GFAT cDNA sequences as probes, candetermine whether particular tumors express or overexpress GFAT.Alternatively, the TaqMan® is used for quantitative RT-PCR analysis ofGFAT expression in cell lines, normal tissues and tumor samples (PEApplied Biosystems).

Various other diagnostic methods may be performed, for example,utilizing reagents such as the GFAT oligonucleotides, and antibodiesdirected against a GFAT, as described above for: (1) the detection ofthe presence of GFAT gene mutations, or the detection of either over- orunder-expression of GFAT mRNA relative to the non-disorder state; (2)the detection of either an over- or an under-abundance of GFAT geneproduct relative to the non-disorder state; and (3) the detection ofperturbations or abnormalities in the signal transduction pathwaymediated by GFAT.

Thus, in a specific embodiment, the invention is drawn to a method fordiagnosing a disease in a patient, the method comprising: a) obtaining abiological sample from the patient; b) contacting the sample with aprobe for GFAT expression; c) comparing results from step (b) with acontrol; and d) determining whether step (c) indicates a likelihood ofdisease. Preferably, the disease is cancer, most preferably a cancer asshown in TABLE 1. The probe may be either DNA or protein, including anantibody.

EXAMPLES

The following experimental section and examples are offered by way ofillustration and not by way of limitation.

I. Drosophila p53 Screen

The Drosophila p53 gene was overexpressed specifically in the wing usingthe vestigial margin quadrant enhancer. Increasing quantities ofDrosophila p53 (titrated using different strength transgenic inserts in1 or 2 copies) caused deterioration of normal wing morphology from mildto strong, with phenotypes including disruption of pattern and polarityof wing hairs, shortening and thickening of wing veins, progressivecrumpling of the wing and appearance of dark “death” inclusions in wingblade. In a screen designed to identify enhancers and suppressors ofDrosophila p53, homozygous females carrying two copies of p53 werecrossed to 5663 males carrying random insertions of a piggyBactransposon (Fraser M et al., Virology (1985) 145:356-361). Progenycontaining insertions were compared to non-insertion-bearing siblingprogeny for enhancement or suppression of the p53 phenotypes. Sequenceinformation surrounding the piggyBac insertion site was used to identifythe modifier genes. Modifiers of the wing phenotype were identified asmembers of the p53 pathway. CG1345 was a suppressor of the wingphenotype. Human orthologs of the modifiers are referred to herein asGFAT.

BLAST analysis (Altschul et al., supra) was employed to identify Targetsfrom Drosophila modifiers. For example, representative sequences fromGFAT, GI#544382 (SEQ ID NO:6), and GI#4826742 (SEQ ID NO:8) share 65%and 66% amino acid identity, respectively, with the Drosophila CG1345.

Various domains, signals, and functional subunits in proteins wereanalyzed using the PSORT (Nakai K., and Horton P., Trends Biochem Sci,1999, 24:34-6; Kenta Nakai, Protein sorting signals and prediction ofsubcellular localization, Adv. Protein Chem. 54, 277-344 (2000)), PFAM(Bateman A., et al., Nucleic Acids Res, 1999, 27:260-2;http://pfam.wus1.edu), SMART (Ponting C P, et al., SMART: identificationand annotation of domains from signaling and extracellular proteinsequences. Nucleic Acids Res. 1999 Jan 1;27(1):229-32), TM-HMM (Erik L.L. Sonnhammer, Gunnar von Heijne, and Anders Krogh: A hidden Markovmodel for predicting transmembrane helices in protein sequences. InProc. of Sixth Int. Conf. on Intelligent Systems for Molecular Biology,p 175-182 Ed J. Glasgow, T. Littlejohn, F. Major, R. Lathrop, D.Sankoff, and C. Sensen Menlo Park, Calif.: AAAI Press, 1998), and dust(Remm M, and Sonnhammer E. Classification of transmembrane proteinfamilies in the Caenorhabditis elegans genome and identification ofhuman orthologs. Genome Res. November 2000;10(11):1679-89) programs. Forexample, the Glutamine amidotransferases class-II domains(PFAM 00310) ofGFATs from GI#s 544382 (SEQ ID NO:6) and 4826742 (SEQ ID NO:8) arelocated at approximately amino acid residues 2-210 and 2-207,respectively. Further, the SIS (Sugar ISomerase) domains (PFAM01380) ofGI#544382 (SEQ ID NO:6) reside at amino acid residues 360 to 494, 531 to667, and the SIS domains of GI#4826742 (SEQ ID NO:8) reside at aminoacid residues 361 to 495, 532 to 668.

II. High-Throughput In Vitro Fluorescence Polarization Assay

Fluorescently-labeled GFAT peptide/substrate are added to each well of a96-well microtiter plate, along with a test agent in a test buffer (10mM HEPES, 10 mM NaCl, 6 mM magnesium chloride, pH 7.6). Changes influorescence polarization, determined by using a Fluorolite FPM-2Fluorescence Polarization Microtiter System (Dynatech Laboratories,Inc), relative to control values indicates the test compound is acandidate modifier of GFAT activity.

III. High-Throughput In Vitro Binding Assay.

³³P-labeled GFAT peptide is added in an assay buffer (100 mM KCl, 20 mMHEPES pH 7.6, 1 MM MgCl₂, 1% glycerol, 0.5% NP-40, 50 mMbeta-mercaptoethanol, 1 mg/ml BSA, cocktail of protease inhibitors)along with a test agent to the wells of a Neutralite-avidin coated assayplate and incubated at 25° C. for 1 hour. Biotinylated substrate is thenadded to each well and incubated for 1 hour. Reactions are stopped bywashing with PBS, and counted in a scintillation counter. Test agentsthat cause a difference in activity relative to control without testagent are identified as candidate p53 modulating agents.

IV. Immunoprecipitations and Immunoblotting

For coprecipitation of transfected proteins, 3×10⁶ appropriaterecombinant cells containing the GFAT proteins are plated on 10-cmdishes and transfected on the following day with expression constructs.The total amount of DNA is kept constant in each transfection by addingempty vector. After 24 h, cells are collected, washed once withphosphate-buffered saline and lysed for 20 min on ice in 1 ml of lysisbuffer containing 50 mM Hepes, pH 7.9, 250 mM NaCl, 20 mM-glycerophosphate, 1 mM sodium orthovanadate, 5 mM p-nitrophenylphosphate, 2 mM dithiothreitol, protease inhibitors (complete, RocheMolecular Biochemicals), and 1% Nonidet P-40. Cellular debris is removedby centrifugation twice at 15,000×g for 15 min. The cell lysate isincubated with 25 μl of M2 beads (Sigma) for 2 h at 4° C. with gentlerocking.

After extensive washing with lysis buffer, proteins bound to the beadsare solubilized by boiling in SDS sample buffer, fractionated bySDS-polyacrylamide gel electrophoresis, transferred to polyvinylidenedifluoride membrane and blotted with the indicated antibodies. Thereactive bands are visualized with horseradish peroxidase coupled to theappropriate secondary antibodies and the enhanced chemiluminescence(ECL) Western blotting detection system (Amersham Pharnacia Biotech).

V. Expression Analysis

All cell lines used in the following experiments are NCI (NationalCancer Institute) lines, and are available from ATCC (American TypeCulture Collection, Manassas, Va. 20110-2209). Normal and tumor tissueswere obtained from Impath, UC Davis, Clontech, Stratagene, and Ambion.

TaqMan analysis was used to assess expression levels of the disclosedgenes in various samples.

RNA was extracted from each tissue sample using Qiagen (Valencia,Calif.) RNeasy kits, following manufacturer's protocols, to a finalconcentration of 50 ng/μl. Single stranded cDNA was then synthesized byreverse transcribing the RNA samples using random hexamers and 500 ng oftotal RNA per reaction, following protocol 4304965 of Applied Biosystems(Poster City, Calif., http://www.appliedbiosystems.com/).

Primers for expression analysis using TaqMan assay (Applied Biosystems,Foster City, Calif.) were prepared according to the TaqMan protocols,and the following criteria: a) primer pairs were designed to spanintrons to eliminate genomic contamination, and b) each primer pairproduced only one product.

Taqman reactions were carried out following manufacturer's protocols, in25 μl total volume for 96-well plates and 10 μl total volume for384-well plates, using 300 nM primer and 250 nM probe, and approximately25 ng of cDNA. The standard curve for result analysis was prepared usinga universal pool of human cDNA samples, which is a mixture of cDNAs froma wide variety of tissues so that the chance that a target will bepresent in appreciable amounts is good. The raw data were normalizedusing 18S rRNA (universally expressed in all tissues and cells).

For each expression analysis, tumor tissue samples were compared withmatched normal tissues from the same patient. A gene was consideredoverexpressed in a tumor when the level of expression of the gene was 2fold or higher in the tumor compared with its matched normal sample. Incases where normal tissue was not available, a universal pool of cDNAsamples was used instead. In these cases, a gene was consideredoverexpressed in a tumor sample when the difference of expression levelsbetween a tumor sample and the average of all normal samples from thesame tissue type was greater than 2 times the standard deviation of allnormal samples (i.e., Tumor−average(all normal samples)>2×STDEV(allnormal samples)).

Results are shown in Table 1. Data presented in bold indicate thatgreater than 50% of tested tumor samples of the tissue type indicated inrow 1 exhibited over expression of the gene listed in column 1, relativeto normal samples. Underlined data indicates that between 25% to 49% oftested tumor samples exhibited over expression. A modulator identifiedby an assay described herein can be further validated for therapeuticeffect by administration to a tumor in which the gene is overexpressed.A decrease in tumor growth confirms therapeutic utility of themodulator. Prior to treating a patient with the modulator, thelikelihood that the patient will respond to treatment can be diagnosedby obtaining a tumor sample from the patient, and assaying forexpression of the gene targeted by the modulator. The expression datafor the gene(s) can also be used as a diagnostic marker for diseaseprogression. The assay can be performed by expression analysis asdescribed above, by antibody directed to the gene target, or by anyother available detection method. TABLE 1 breast . . colon . . kidney .. lung . . ovary . GI#4503980 (SEQ ID NO: 2) 2 12 . 7 30 . 0 0 . 5 14 .2 7 GI#4826741 (SEQ ID NO: 3) 1 12 . 14 30 . 0 0 . 4 14 . 0 7

1. A method of identifying a candidate p53 pathway modulating agent,said method comprising the steps of: (a) providing an assay systemcomprising a purified GFAT polypeptide or nucleic acid or a functionallyactive fragment or derivative thereof; (b) contacting the assay systemwith a test agent under conditions whereby, but for the presence of thetest agent, the system provides a reference activity; and (c) detectinga test agent-biased activity of the assay system, wherein a differencebetween the test agent-biased activity and the reference activityidentifies the test agent as a candidate p53 pathway modulating agent.2. The method of claim 1 wherein the assay system comprises culturedcells that express the GFAT polypeptide.
 3. The method of claim 2wherein the cultured cells additionally have defective p53 function. 4.The method of claim 1 wherein the assay system includes a screeningassay comprising a GFAT polypeptide, and the candidate test agent is asmall molecule modulator.
 5. The method of claim 4 wherein the assay isa transferase assay.
 6. The method of claim 1 wherein the assay systemis selected from the group consisting of an apoptosis assay system, acell proliferation assay system, an angiogenesis assay system, and ahypoxic induction assay system.
 7. The method of claim 1 wherein theassay system includes a binding assay comprising a GFAT polypeptide andthe candidate test agent is an antibody.
 8. The method of claim 1wherein the assay system includes an expression assay comprising a GFATnucleic acid and the candidate test agent is a nucleic acid modulator.9. The method of claim 8 wherein the nucleic acid modulator is anantisense oligomer.
 10. The method of claim 8 wherein the nucleic acidmodulator is a PMO.
 11. The method of claim 1 additionally comprising:(d) administering the candidate p53 pathway modulating agent identifiedin (c) to a model system comprising cells defective in p53 function and,detecting a phenotypic change in the model system that indicates thatthe p53 function is restored.
 12. The method of claim 11 wherein themodel system is a mouse model with defective p53 function.
 13. A methodfor modulating a p53 pathway of a cell comprising contacting a celldefective in p53 function with a candidate modulator that specificallybinds to a GFAT polypeptide comprising an amino acid sequence selectedfrom group consisting of SEQ ID NOs: 6, 7, or 8, whereby p53 function isrestored.
 14. The method of claim 13 wherein the candidate modulator isadministered to a vertebrate animal predetermined to have a disease ordisorder resulting from a defect in p53 function.
 15. The method ofclaim 13 wherein the candidate modulator is selected from the groupconsisting of an antibody and a small molecule.
 16. The method of claim1, comprising the additional steps of: (d) providing a secondary assaysystem comprising cultured cells or a non-human animal expressing GFAT,(e) contacting the secondary assay system with the test agent of (b) oran agent derived therefrom under conditions whereby, but for thepresence of the test agent or agent derived therefrom, the systemprovides a reference activity; and (f) detecting an agent-biasedactivity of the second assay system, wherein a difference between theagent-biased activity and the reference activity of the second assaysystem confirms the test agent or agent derived therefrom as a candidatep53 pathway modulating agent, and wherein the second assay detects anagent-biased change in the p53 pathway.
 17. The method of claim 16wherein the secondary assay system comprises cultured cells.
 18. Themethod of claim 16 wherein the secondary assay system comprises anon-human animal.
 19. The method of claim 18 wherein the non-humananimal mis-expresses a p53 pathway gene.
 20. A method of modulating p53pathway in a mammalian cell comprising contacting the cell with an agentthat specifically binds a GFAT polypeptide or nucleic acid.
 21. Themethod of claim 20 wherein the agent is administered to a mammaliananimal predetermined to have a pathology associated with the p53pathway.
 22. The method of claim 20 wherein the agent is a smallmolecule modulator, a nucleic acid modulator, or an antibody.
 23. Amethod for diagnosing a disease in a patient comprising: (a) obtaining abiological sample from the patient; (b) contacting the sample with aprobe for GFAT expression; (c) comparing results from step (b) with acontrol; (c) determining whether step (c) indicates a likelihood ofdisease.
 24. The method of claim 23 wherein said disease is cancer. 25.The method according to claim 24, wherein said cancer is a cancer asshown in Table 1 as having >25% expression level.